Lithographic processes are commonly used to produce a pattern in a photoresist film formed on a substrate to develop a desired integrated circuit product. A variety of systems are known for accomplishing such a result. Principal factors for determining the quality (primarily the shape) of the produced image include "exposure" (illumination time and intensity) and "focus" (the position of the photoresist film relative to the focal plane of the imaging system).
To ensure that the patterns being produced are kept within acceptable tolerances, a common practice is to monitor (measure) the dimensions of the patterns to account for potential variations in processing, primarily resulting from variations in exposure and focus. The importance of such monitoring increases as the size of the features being produced decreases. The difficulty in monitoring such features also increases, however, as the size of the features decreases. This difficulty is exacerbated for features having a size on the order of one micron or less. This is because the use of a scanning electron microscope (SEM) to perform such inspections, as is generally preferred, tends to be relatively slow in operation and difficult to automate for features of a smaller size. The use of optical tools would permit faster and more readily automated operations to be implemented, but such optical techniques were generally considered to be inadequate to resolve features of a smaller size, particularly those having dimensions of less than about one micron.
To overcome this problem, U.S. Pat. No. 5,629,772 (issued to Ausschnitt) discloses an optical system capable of resolving features of a smaller size for measuring the bias (i.e., variation) of a minimum feature in a lithographic process. To this end, an array of elements having a width and space corresponding to the minimum feature is created. Length changes of an element in the array (resulting from image-shortening effects due to the lithographic process) are then measured and the bias of the element is calculated in the width dimension. A test site having groups of elements in the array is then defined to facilitate automatic bias measurement of array lengths and separations, allowing the use of optical (non-SEM) tools which would otherwise have been incapable of measuring the minimum feature widths being monitored.
In practice, however, difficulties were encountered when monitoring minimum features using the techniques disclosed in U.S. Pat. No. 5,629,772 (i.e., an end-of-line monitoring, or metrology, of a nested line pattern developed in the test site defined for the grouped array of elements). For example, a key problem encountered was the interaction of sublayer films with the measurement structure. A typical overall measurement structure might include a lithographically formed, nested line pattern 1 developed in the photoresist 2, on top of a series of thin sublayer films 3, 4 such as silicon dioxide or silicon nitride. Sublayer films 3, 4 are formed on a substrate 7 such as a silicon wafer. Such a structure is shown in FIGS. 1A and 1B.
The underlying films 3, 4 can bias the value of the line-shortening measurement due to optical interactions between the photoresist line pattern 1 and the sublayer films 3, 4, as shown in FIG. 2. As the thickness of the sublayer films 3, 4 is varied, the reflectivity of the measurement target varies, in turn biasing the measurement of line length for the nested features. The ideal detection scheme would therefore permit the detection of light reflected from the nested line pattern 1 while rejecting the light generated in the underlying films 3, 4.
One method that can be used to reject extraneous light, in the context of an optical alignment of lithographic features, is described by N. Bobroff et al., "Alignment Errors From Resist Coating Topography," J. Vac. Sci. Technol. B, Vol. 6, No. 1, pp. 403-08 (January/February 1988). The disclosed method uses darkfield illumination for lithographic alignment. With reference to FIG. 3, darkfield illumination 5 allows the rejection of the specular component 6 of reflected intensity, in this way minimizing interaction of the substrate films 3, 4 with lithographic alignment. Another method that can be used to reject extraneous light is described in U.S. Pat. No. 4,779,001 (issued to Makosch). In this case, the disclosed system uses polarized light with a single wavelength for illumination in the context of optical alignment performed in conjunction with a pair of nested line gratings. In practice, however, neither of these systems has proven to be entirely satisfactory.
Therefore, the primary object of the present invention is to provide a system for effectively monitoring a nested line pattern developed in a defined test site using an optical tool to detect line shortening for purposes of optical width measurement. Another object of the present invention is to provide a system for monitoring a nested line pattern developed in a defined test site which eliminates extraneous light from the sublayer thin films associated with the substrate, to provide greater insensitivity to the substrate films. Still another object of the present invention is to provide a system for monitoring a nested line pattern developed in a defined test site which permits the use of broadband illumination to permit imaging of the defined test site over a broad range of wavelengths.